One objection to the three-box model of memory is that a. short-term memory is not usually involved in the conscious processing of information b. the brain performs many independent operations simultaneously c. sensory register is actually able to store information for 30 seconds d. there is a limit to the capacity of long-term memory

b. the brain performs many independent operations simultaneously

Explanation:

Atkinson and Shiffrin´s three-box model of memory proposed that human memory was as a system with three separated stages:  the sensory registers, short-term memory, and long-term memory. However, it has been proven that there are multiple memory systems working in parallel, and each has short-term and long-term operations.